The present invention relates generally to sealed bearing earth boring drill bits, such as rotary cone rock bits. More particularly, the invention relates to drill bits that have a dual seal arrangement for protecting internal bearing elements. Yet more particularly, the present invention relates to providing for pressure communication between the interior and exterior of earth boring dual-seal drill bits.
During earthen drilling operations with the use of sealed bearing drill bits, such as rotary cone drill bits, it is necessary to protect the bearing elements of the bit from contamination in order to sustain bit operability. In particular, it is desirable to isolate and protect the bearing elements of the bit, such as bearings, bearing lubricant and bearing surfaces that are located in a bearing cavity or cavities between each corresponding bit leg and roller cone, from earthen cuttings, mud and other debris in the drilling environment. Introduction into the bearing system of such contaminants can lead to deterioration of the bearing lubricant, bearings and bearing surfaces, causing premature bit failure. It is well known in the art to provide an annular seal around the bearing elements to prevent contamination thereof by particles entering through the annular opening and into the gap that is formed between each leg and corresponding roller cone and that extends to the bearing cavity.
In a downhole drilling environment, the borehole contains "drilling fluid," which can be drilling mud, other liquids, air, other gases, or a mixture or combination thereof. In the typical liquid drilling environment of a petroleum well, the downhole fluid pressure at the location of the drill bit, the "external pressure," can be very high and fluctuating. At the same time, internal pressure within the bearing cavity, the "internal pressure," can also be very high and fluctuating due, for example, to thermal expansion and out-gassing of lubricant in the bearing cavity, and cone movement relative to the leg. These high pressure changes internal and external to the bearing cavity may cause a differential pressure across the bearing seal, thus resulting in a major load on the seal. When the internal pressure is greater than the external pressure, the seal may be drawn to and possibly extruded into the gap. Likewise, a greater external pressure can cause the seal to be drawn in the direction of the bearing cavity and possibly extruded therein. This may cause excessive wear to the seal and eventual bit inoperability. Furthermore, when the pressure differential reaches a certain level in each above scenario, the seal can leak, allowing lubricant to pass from the bearing cavity into the gap in the first scenario, and drilling fluid to pass from the gap into the bearing cavity in the second scenario.
Generally, when the internal pressure and the external pressure are equal, the differential pressure across the bearing cavity seal will be zero. There will be no pressure to force the drilling fluid or lubricant by the seal, or to force the seal into the gap or bearing cavity. Thus, it is generally desirable to achieve or maintain a differential pressure of approximately zero. In the prior art, a lubricant reservoir system having a flexible diaphragm located in a lubricant reservoir cavity in the bit leg is used to equalize the internal and external pressure. The flexible diaphragm separates the internal lubricant from the external drilling fluid and communicates the external pressure to the portion of the bearing seal adjacent to the bearing cavity. This type of pressure compensation system for a single seal bit is schematically shown in FIG. 1a.
Referring to FIG. 1a, when the external, or borehole, pressure Pd of the drilling fluid in the borehole B.sub.1 increases and is greater than the internal pressure Pg in the bearing cavity, the seal S.sub.1 will be forced inwardly toward the bearing cavity B.sub.2. With the use of a flexible diaphragm D.sub.1, the external pressure Pd is also applied to the diaphragm D.sub.1, which transmits the pressure Pd, equalizing it with the internal pressure Pg. As a result, the pressure on both sides of the seal S.sub.1 is balanced, preventing the occurrence of any differential pressure across the seal S.sub.1. Similarly, when the pressure Pg increases, Pg will temporarily be larger than Pd, causing the diaphragm D.sub.1 to expand outwardly to increase the internal volume of the bearing cavity B.sub.2. As the internal volume increases, the internal pressure Pg will decrease. Pg will drop to equilibrium with Pd, and the internal volume will stop increasing.
Dual seal arrangements have been proposed having an outer seal around a primary inner seal. The purpose of including a second seal is typically to provide a second layer of protection from particles entering the gap through the annular opening. When an outer seal is added, it may be necessary, such as in drill bits used for petroleum wells, that the bit be capable of compensating for the differential pressure across both seals. FIG. 1b shows a two-seal schematic with both seals providing substantially absolute seals, the "space" Sp formed between the seals S.sub.1, S.sub.2 being completely filled with incompressible fluid, and there being no variation in the density of the incompressible fluid. In this scenario, the incompressible fluid in space S.sub.p between the seals S.sub.1, S.sub.2 acts like a rigid body that transmits pressure from Pg.sub.1, which is the (internal) bearing cavity pressure, to Pd and from Pd to Pg.sub.1. For example, when the external fluid pressure Pd increases, diaphragm D.sub.1 will be pushed inwardly, causing the internal pressure Pg.sub.1 to equal the external pressure Pd. Because the fluid between seals S.sub.1 and S.sub.2 is incompressible, it will transmit the increased pressure between S.sub.1 and S.sub.2 and neither seal S.sub.1 or S.sub.2 will be displaced.
However, during borehole drilling operations, such as with rotary cone sealed bearing drill bits, various factors will alter ideal conditions and require something more to equalize the differential pressure across both seals S.sub.1 and S.sub.2. For example, there is relative movement between the roller cone and bit leg, which causes the volume of the space S.sub.p between the seals S.sub.1 and S.sub.2 to significantly increase and decrease. A change in the volume of the space S.sub.p will change the chamber pressure Pg.sub.2 in the space Sp, causing conditions where Pg.sub.2 &gt;Pd, Pg.sub.1 upon contraction of the space Sp, and where Pg.sub.2 &lt;Pd, Pg.sub.1 upon expansion of the space Sp. Thus, there will be differential pressures across both seals S.sub.1, S.sub.2, causing their movement and possible extrusion, which can cause accelerated seal wear and eventual bit failure.
Another potential factor altering ideal conditions is the thermal expansion, or out-gassing, of the incompressible fluid between the seals S.sub.1, S.sub.2 due to elevated temperatures within the bit. Referring to FIG. 1b, expansion of the incompressible fluid in the space Sp between the seals S.sub.1, S.sub.2 will elevate the chamber pressure Pg.sub.2. Increasing the chamber pressure Pg.sub.2 can cause a differential pressure across the seals S.sub.1, S.sub.2 such that Pg.sub.2 &gt;Pd, Pg.sub.1, which can result in accelerated wear and possible extrusion of seals S.sub.1, S.sub.2. Still another factor is the existence of air trapped in the space Sp between the seals S.sub.1, S.sub.2. In this instance, the mixture of air and fluid in space Sp is not incompressible. When external pressure Pd increases, Pg.sub.1 will eventually equal Pd due to the diaphragm D.sub.1, but Pd&gt;Pg.sub.2 and Pg.sub.1 &gt;Pg.sub.2 because of the presence of air in the space Sp between the seals S.sub.1, S.sub.2. The chamber pressure Pg.sub.2 in the space Sp will not increase until the seals S.sub.1, S.sub.2 move closer together and the air volume in space Sp decreases. This differential pressure across seals S.sub.1, S.sub.2 will cause the movement and possible extrusion of the seals into the space Sp and excessive wear on the seals.
In the prior art, U.S. Pat. No. 5,441,120, which is hereby incorporated by reference herein in its entirety, discloses the use of an additional flexible diaphragm D.sub.2, such as shown in FIG. 1c herein, to attempt to equalize, or balance the chamber pressure Pg.sub.2 of the space Sp with the external pressure Pd or internal pressure Pg.sub.1. Further increases in external pressure Pd will thereafter be transmitted through the fluid in the space Sp. Such a system has various disadvantages. For example, a system made in accordance with U.S. Pat. No. 5,441,120 requires or occupies much space within the bit leg, structurally weakening the bit. For another example, such a system does not allow for pressure relief from the space Sp, such as caused by thermal expansion and outgassing of the incompressible fluid between the seals S.sub.1, S.sub.2, which can cause damage to the seals as described above. It should be understood that there are other disadvantages and features of the disclosure of U.S. Pat. No. 5,441,120 as well as various features of the invention of each claim herein that distinguish one from the other. Thus, in any comparison, the disclosure of U.S. Pat. No. 5,441,120 should be compared as a whole to the claimed invention of any particular claim herein as a whole to distinguish them.
U.S. Pat. Nos. 4,981,182 and 5,027,911, which are also hereby incorporated herein in their entireties, disclose various embodiments of drill bits including inner and outer seals and where lubricant is bled out of the bit past the outer seal to prevent drilling debris from accumulating and damaging the inner and outer seals. In some such embodiments, passages in the bit allow lubricant to travel from the bearing cavity to the space between the seals. In other embodiments, a hydrodynamic inner seal is used, which allows the leakage of lubricant from the bearing cavity to the space between the seals. In both instances, the pressure of the lubricant presumably forces the outer seal to open and allow the bleeding of lubricant from the bit. These systems also have various disadvantages. For example, the continuous bleeding of lubricant past the outer seal (particularly if the outer seal fails) can lead to the depletion of bearing lubricant in the bit, and cause bearing and bit damage due to a lack of lubricant. For another example, if the space between the seals in such configurations is not filled with lubricant, such as which will occur if there is a decrease or stoppage in the flow of lubricant from the bearing cavity to the space, a high pressure differential across the seals can result, causing damage to the seals as described above. For yet another example, with many such embodiments, because the space between the seals and the bearing cavity are in fluid communication, there exists the possibility that debris or drilling fluid bypassing the outer seal, such as when the outer seal fails, will move through the space between the seals and into the bearing cavity, causing contamination and damage to therein and to the bearing elements. It should be understood that there are other disadvantages and features of the disclosures of U.S. Pat. Nos. 4,981,182 and 5,027,911 as well as various features of the invention of each claim herein that distinguish them. Thus, in any comparison of U.S. Pat. Nos. 4,981,182 or 5,027,911 and any claim herein, such disclosure should be compared as a whole to the claim as a whole to distinguish them.
Thus, there remains a need for improved techniques and mechanisms for substantially balancing or minimizing the pressure differential upon the primary and secondary seals of a dual seal configuration, particularly by allowing pressure communication between the interior and exterior of the drill bit. Ideally, the devices and techniques will accommodate cone movement, thermal expansion of the fluid and/or out-gassing between the primary and secondary seals, and trapped air in the space between the seals. Especially well received would be pressure communication devices that do not require substantial additional components, large space requirements in the bit, or highly complex manufacturing requirements for the bit. Also well received would be a pressure communication technique and device that will prevent the pressure differential across the dual seals from exceeding an upper limit, such as, for example, 100 psi. It would also be advantageous to include the use of an incompressible fluid having the capabilities of retaining sufficient viscosity to act as a medium for the transmission of energy between the primary and secondary seals, of retaining its lubrication properties, and/or of slowing the intrusion of abrasive particles to the primary seal--when and after the incompressible fluid is exposed to drilling fluid. These and other needs in the art will become apparent to those of skill in the art upon review of this patent specification, claims and drawings.